Q: Mac Pro won't restart

What symbol comes up?

About the screens you see when your Mac starts up - Apple Support

Without knowing that all I can suggest is:

- Try resetting the SMC and NVRAM/PRAM

Intel-based Macs: Resetting the System Management Controller (SMC)

About NVRAM and PRAM       

- Try starting in Safe Mode

OS X: What is Safe Boot, Safe Mode?

- Boot to Recovery and repair the startup disk

OS X: About OS X Recovery - Apple Support

- If it is repairable reinstall the OSX

How to reinstall OS X on your Mac - Apple Support

- If you do not have a backup use disk utility to restore the internal disk to an external disk so you can try to recover data.

Then format the startup disk and do a fresh install of OSX